1. Field of the Invention
This invention relates generally to a torque converter for an automatic transmission, and, in particular, to control of an impeller clutch among locked released and slipping states.
2. Description of the Prior Art
A torque converter is a modified form of a hydrodynamic fluid coupling, and like a fluid coupling, is used to transfer rotating power from a prime mover, such as an internal combustion engine or electric motor, to a rotating driven load. A torque converter is able to multiply torque when there is a substantial difference between input and output rotational speed, thus providing the equivalent of a reduction gear.
In a torque converter there are at least three rotating elements: the impeller, which is mechanically driven by the prime mover; the turbine, which drives the load; and the stator, which is interposed between the impeller and turbine so that it can alter oil flow returning from the turbine to the impeller. The classic torque converter design dictates that the stator be prevented from rotating under any condition, hence the term stator. In practice, however, the stator is mounted on an overrunning clutch, which prevents the stator from counter-rotating the prime mover but allows for forward rotation.
Pumping losses within the torque converter reduce efficiency and generate waste heat. In modern automotive applications, this problem is commonly avoided by use of a lock-up clutch, which physically links the impeller and turbine, effectively changing the converter into a purely mechanical coupling. The result is no slippage, and therefore virtually no power loss and improved fuel economy.
While torque multiplication increases the torque delivered to the turbine output shaft, it also increases the slippage within the converter, raising the temperature of the fluid and reducing overall efficiency. For this reason, the characteristics of a torque converter must be carefully matched to the torque curve of the power source and the intended application. Changing the blade geometry of the stator and/or turbine will change the torque-stall characteristics, as well as the overall efficiency of the unit. Highway vehicles generally use low stall torque converters to limit heat production, and provide a more firm feeling to the vehicle's characteristics.
In a three-pass converter control system, the ability to rapidly and precisely control converter hydraulic system resistance through the converter discharge circuit controls an impeller clutch, which enables neutral idle, variable K-curve and lower load on the transmission oil pump. Without control of the converter discharge circuit it would not be possible to control the impeller clutch, which enables idle disconnect and variable K-curve.
In a conventional three-pass torque converter hydraulic control system, converter charge pressure and converter by-pass pressures are controlled by a valve or multiple valves in the main control. Converter charge pressure and by-pass pressure are either based on a percentage of regulated line pressure to prevent cover ballooning of the housing, or in the case of a closed piston, regulated maximum line pressure to optimize converter clutch torque capacity. Solenoids are used as the hydraulic control device. Depending on the precision of control, a pulse width modulated (PWM) or variable force solenoid (VFS) can be used. Converter discharge pressure is generally not separately controlled, but rather in a conventional converter is a dependent variable or a direct function of converter charge pressure and a restriction in the exit passage of the torque converter, stator, main control, case, and cooling circuit. Historically only converter charge pressure and by-pass pressure have been controlled. There has been little practical reason to precisely control and vary converter discharge resistance and pressure.
There is a need in the industry to control an impeller clutch in a torque converter during engine idle conditions in order to disconnect the engine and impeller and to vary the torque converter's K-curve.